What are inferior leads on an electrocardiogram (ECG)?

Inferior Leads on ECG

The inferior leads on an electrocardiogram (ECG) are leads II, III, and aVF, which view the inferior wall of the left ventricle and can detect pathology such as inferior wall myocardial infarction. 1

Anatomical Significance

• Leads II, III, and aVF have their positive poles oriented inferiorly, allowing them to detect electrical activity in the inferior wall of the left ventricle 1
• These leads primarily reflect the electrical activity of the inferior (diaphragmatic) surface of the heart 1
• The inferior leads are supplied primarily by the right coronary artery (RCA) in approximately 80% of people, or by the left circumflex artery (LCx) in approximately 20% of people (dominant circulation) 1

Clinical Significance in Myocardial Infarction

• ST-segment elevation in leads II, III, and aVF indicates inferior wall myocardial infarction 1
• When the RCA is the culprit vessel, ST elevation is typically greater in lead III than in lead II 1, 2
• RCA occlusion often produces reciprocal ST-segment depression in leads I and aVL (which are oriented superiorly and leftward) 1, 3
• ST depression in aVL is a sensitive marker for acute inferior myocardial infarction, sometimes appearing before obvious ST elevation in the inferior leads 3, 4

Differentiating RCA vs. LCx Occlusion

• When ST elevation in lead III is greater than in lead II, this suggests RCA occlusion rather than LCx occlusion 2, 5
• ST depression in aVL that is greater than ST depression in lead I also suggests RCA occlusion 5
• The ratio of ST depression in V3 to ST elevation in III ≤1.2 is another indicator of RCA occlusion 5

Right Ventricular Involvement

• Proximal RCA occlusion may cause right ventricular infarction, which directs the ST vector rightward, anteriorly, and inferiorly 1
• This results in ST elevation in right-sided chest leads V3R and V4R, and often in lead V1 1, 2
• Right-sided leads (V3R, V4R) should be recorded as quickly as possible after symptom onset, as ST elevation in these leads is transient compared to the ST elevation in the inferior leads 1

Posterior Wall Involvement

• ST depression in leads V1-V3 accompanying inferior MI may represent posterior (now termed lateral or inferolateral) wall involvement 1
• Additional posterior leads (V7-V9) may be needed to confirm posterior wall involvement 2

Clinical Pitfalls and Recommendations

• The American Heart Association recommends recording right-sided chest leads V3R and V4R in all patients with ECG evidence of acute inferior wall ischemia/infarction 1
• ECG machines should be programmed to suggest recording right-sided chest leads when ST elevation >0.1 mV is detected in leads II, III, and aVF 1
• Serial ECGs are important in patients with chest pain and high clinical suspicion for acute MI, as early reciprocal changes in lead aVL may precede obvious ST elevation in the inferior leads 4

Remember that inferior wall MI may be associated with various complications including AV blocks, right ventricular infarction, and posterior wall extension, each with specific hemodynamic abnormalities and increased mortality 6.